Superconducting ceramic film-forming paste

Abstract

 A paste for forming a superconducting ceramic film comprises a powder of a superconducting ceramic material or powders of ingredients which form a superconductive ceramic material by firing, an organic binder and a solvent. The paste is to be utilized in producing a multi-layer superconducting circuit substrate formed of a plurality of insulating layers laminated together and having holes penetrated through the layers with interconnection layers of the superconductive ceramic material being located between the insulating layers and the through holes in the insulating layers being filled with superconductive ceramic material electrically connecting the interconnection layers. The superconductive ceramic material is preferably isolated from the insulating layers by noble metal layers encapsulating the superconductive ceramic material. The superconductive ceramic material itself is either an oxide ceramic material represented by one of the general formulae X_aY_1-aZO_b and (X_1-aY_a)₂ZO_b, where O < a < 1; 3 b b <4; X is at least one element selected from the group of Ba, Ca, Sr and Mg; Y is at least one element selected from the group of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and Z is at least one element selected from the group of Cu, Ag, and Au; or is one of the superconducting oxides of the Bi-Sr-Ca-Cu-O and TI-Bi-Sr-Ca-Cu-O systems.

Description

[0001] The present invention relates to the production of a multi-layer superconducting circuit substrate, particular a multi-layer superconducting circuit substrate suitable for use in a high speed computer such as a super computer, and specifically to a paste for forming a superconducting ceramic film comprised by such superconducting circuit substrate. A substrate for high speed processing requires a conductive material having a low electrical resistance, in addition to an insulating material having a low dielectric constant, of the substrate, to ensure an efficient transmission of signals

[0002] As a substrate for a high speed processing, a multi-layer ceramic substrate is known in which a glass- ceramic material, i.e., a complex of a ceramic and a glass (dielectric constant ∈ ≒ 5), is used as an insulating material and copper (resistivity p = 1.7 Ω·cm) is used as a conducting material (for example, see Yogyo Kyokai Annual Conference Digest, 1985, P. 533).

[0003] Japanese Unexamined Patent Publication (Kokai) No. 60-173885, published on November 7, 1985, describes a superconductive oxide material of BaBiO_3-δ (0 ≦ δ < 0.5) wherein less than 50% by atom of Ba is replaced by Sr, Pb, Sc, Y or lanthanide element, the superconductive oxide material having a superconducting transition temperature of about 12 K; and a process for manufacturing a superconductive oxide material of BaBiO_3-δ (0 ≦ δ < 0.5); the process comprising uniformly mixing a Ba compound and a Bi compound, calcining the mixture in an oxidizing atmosphere at a temperature of more than 600°C, pulverizing the calcined product, pressing the resultant powder to form a body, and annealing the body in on atmosphere containing more than 10 Kg/cm² of oxygen at 700 °C to compensate for the large amount of oxygen lost in the calcining step.

[0004] Also known are the following oxide superconductive materials:

(1) La-Ba-Cu-O system:

Bednorz: Z. Phys. B. 64, 189, 1986;

Uchida: J. J. A. P. vol 26, No. 1, L1, 1987;

Chu: Physical Review Letters; vol. 58, No. 4, 405, 1987;

Jorgensen: Physical Review Letters; vol. 58, No. 10, 1024, 1987;

(2) La-Sr-Cu-O system:

Cava: Physical Review Letters: vol. 58, No. 4, 408, 1987;

Mattheiss: Physical Review Letters; vol. 58, No. 10, 1028, 1987; and

(3) Y-Ba-Cu-0 system:

Hor: Physical Review Letters; vol 58, No. 9, 911, 1987; and

Wu: Physical Review Letters; vol. 58, No. 9, 908, 1987.

[0005] Specifically:

(1) the La-Ba-Cu-O system includes

(2) the La-Sr-Cu-O system includes

La_1.9Sr_0.1 Cu0₄

La_1.8Sr_0.2CuO₄ : T_cf 28.5K, T_cm 33.1 K, Too 36.5K;

La_1.7Sr_0.3CuO₄ : T_cm 15K, T_co 35K;

La_1.925Sr_0.075CuO₄: T_cf 26.5K, T_cm 34K, T_co 52K,

La_1.85Sr_0.15CuO₄ : T_cf 36K, t_cm 37.5K, Too 40K;

(3) the Y-Ba-Cu-0 system includes

(Y_0.6Ba_0.4)₂ CuO_4-δ T_cf 88.5K, >77K, T_co ≧ 100K;

Y-Ba-Cu-0 T_c 77K, T_co 100K;

(by Keiichi Ogawa)

Y-Ba-Cu-0 T_e 95K;

(by Molecular Institute and Tohoku University)

Y_o.₄Ba_o.₆Cu0₃ T_e 93, T_ea 123K.

(by Kazumasa Togano)

[0006] Other superconducting materials of ceramic nature and belonging to various oxide systems have subsequently become known. For example, Physical Review Letters, Vol. 58, No. 9, 2nd March 1987 at pages 911-912 discusses the relative merits of Y-Ba-Cu-O, La-Ba-Cu-O and La-Sr-Cu-O systems.

[0007] At present, the conducting materials used in substrates have an electrical resistance which inhibits the design of even longer or finer interconnection patterns. A demand therefore exists for a substrate for high speed processing, the substrate incorporating a conductivity material having an even lower electrical resistance, which substrate is to be used in a future generation computer. Although a superconductive oxide material meets this requirement , i.e., a lower electrical resistance, it is difficult to form a superconductive oxide material into an interconnection pattern. New interconnection patterns should have definite peripheries which are at least as definite as those of the copper patterns, in order to reduce interactions between neighboring patterns, since the density of the patterns will be made higher.

[0008] The use of superconductor materials has developed to the extent that they have nevertheless been described for use in multi-layer glass ceramic modules for Josephson devices in IBM Technical Disclosure Bulletin, Vol. 27, No. 7B, Dec. 1984 at page 4254. For this purpose superconducting wiring of, for example, Nb₃Sn is screen printed on individual green sheets of an insulating glass ceramic composition which are laminated together and fired in a neutral or reducing atmosphere at a temperature at which the glass ceramic composition sinters and the Nb and Sn particles diffuse and react to form the NbaSn intermetallic.

[0009] Moreover, EP-A-0 285 445 having a publication date of 5th October 1988 and based, inter alia, on priority application JP 81487/87 dated 1st April 1987 discloses a superconductor device in which a superconductor of a ceramic material is formed on a substrate, with the lower surface of the superconductor being in contact with a non-oxide material such as a nitride material or carbon compound material. There may be a plurality of the semiconductor elements laminated together with there being electric wiring between the semiconductor elements themselves.

[0010] We have been investigating, in the meanwhile, the use of superconducting materials, particularly of ceramic nature, especially oxide systems in multi-layer glass ceramic modules, wherein, as in the aforementioned IBM Technical Disclosure Bulletin reference already mentioned herein, the glass layers have an insulating function in relation to the superconductive ceramic material. We have found, however, that ingredients of the superconductive ceramic material and the insulating material or diffusion of ingredients of the superconductive ceramic material into the insulating material causes a deterioration of the superconductive property of the superconductive ceramic material for reasons to be detailed hereinafter.

[0011] With a view to providing a multi-layer substrate which comprises a conducting pattern of superconducting ceramic material having a definite periphery and a low electrical resistance which does not deteriorate during manufacture of the substrate, there is disclosed in our EP-A-0 292 125 from which this application is divided, a multi-layer substrate comprising:

a plurality of insulating layers laminated into a multi-layer structure, the insulating layers having holes penetrated therethrough;

interconnection layers of a superconductive ceramic material located between the insulating layers; and

the through-holes in the insulating layers being filled with superconductive ceramic material electrically connecting said interconnection layers.

[0012] Thus, according to the disclosure of EP-A-0 292 125, not only are the copper interconnection patterns in a multi-layer substrate of the prior art replaced by interconnection patterns of a superconductive nature but the latter are of a ceramic nature being generally formed of oxide superconductive material including Y-Ba-Cu-O, La-Sr-Cu-O and Sc-Ba-Cu-O systems which exhibit superconductivity at a temperature above the boiling point of nitrogen, i.e. 77K.

[0013] According to the invention of EP-A-0 292 125 the superconductive ceramic material of the interconnection layers and in the through-holes is isolated from the insulating layers by metal layers, the metal layers being of a noble metal and encapsulating the superconductive ceramic material.

[0014] It is an object of this invention to provide a paste for forming a superconducting ceramic film to be comprised by such multi-layer substrate.

[0015] According to the present invention, there is provided a paste for forming a superconducting ceramic film, comprising a powder of a superconducting ceramic material or powders of ingredients which form a superconductive ceramic material by firing, an organic binder and a solvent, characterized in that the superconductive ceramic material is an oxide ceramic material represented by one of the general formulae X_aY₁-_aZO_b and (X_1-aY_a)₂ZO_b, where O < a < 1; 3 b b <4; X is at least one element selected from the group of Ba, Ca, Sr and Mg; Y is at least one element selected from the group of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and Z is at least one element selected from the group of Cu, Ag, and Au.

[0016] X, Y, and Z in the above formula may be more than one element. For example, X may be Ba-Ca-Sr, Ba-Ca-Mg, Ba-Ca, or Ca-Mg; Y may be Sc-Y, Sc-Y-La, Sc-Y-La-Ce, or Pr-Nd; and, Z may be Cu-Ag, Cu-Au, Ag-Au, or Cu-Ag-Au.

[0017] Thus, the oxide superconductive material may be, for example, (Ba_0.6Ca_0.2Sr_0.2)_0.6(Sc_0.7Y_0.2La_0.1)_0.4(Cu_0.8Ag_0.2)₃O_4-δ or (Ba_0.8Mg_0.2)_0.7(Sc_0.6Nd_0.4)_0.3Cu₃O_4-δ-

[0018] A preferred oxide superconductive material is


wherein M" stands for at least one of Ba, Sr, Ca, and Mg; M^III stands for at least one of Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, and Lu; 0.5 ≦ x 0.9; 1 < y 2;
1 < z 2; and 6 stands for a deviation of the amount of oxygen from stoichiometry; the oxide superconducting material exhibiting superconductivity above the boiling point of nitrogen (77K).

[0019] Another example of a superconducting ceramic material is a bismuth-system (Bi-Sr-Ca-Cu-O system) superconducting ceramic material. The exact chemical formula of the bismuth-system superconducting ceramic material is not known but can be formed from a starting material of a mixture of Bi, Sr, Ca, and Cu compounds in a molar ratio (based on these elements) of 0.25-2:0.1-5:0.1-5:0.5-4. For example, 1:1:1:2; 1:1:1:3, 4:3:3:6; and 4:3:3:4. Further, another superconducting ceramic material can be formed from a starting material of a mixture of TI, Bi, Sr, Ca, and Cu compounds in a molar ratio (based on these elements) of 0.25-2:0.25-2:0.1-5:0.1-5:0.5-4. These superconducting ceramic materials exhibit superconductivity above the boiling point of nitrogen (77K).

[0020] Typically, the paste comprises 100 parts by weight of the above inorganic powder or powders, 0.5 to 10 parts by weight of the organic binder; and 5 to 10 parts by weight of the non-volatile solvent.

[0021] In the multi-layer substrate of EP-A-0292 125, the interconnection layers of a superconductive ceramic material are generally isolated from ceramic layers by a noble metal preferably gold, silver, platinum or an alloy thereof, to prevent a reaction of the superconductive ceramic material, with the insulating material of the insulating layers sandwiching the superconductive ceramic material during firing of the green sheets of the insulating material or annealing of the ceramic superconductive material. Ingredients of the superconductive ceramic material diffuse into the insulating material during firing or annealing. The reaction between the superconductive ceramic material and the insulating material or diffusion of the ingredients of a superconductive ceramic material into the insulating material causes a deterioration of the superconductive property of the superconductive ceramic material. The noble metal isolating a superconductive ceramic layer from an insulating layer is preferably gold, silver, platinum or an alloy thereof, since it must withstand firing or annealing in an oxygen-containing atmosphere it about 1000°C. An oxygen-containing atmosphere and a temperature of about 1000 °C are necessary to preserve or recover the superconductive property of a superconductive ceramic material. The metal may supplement the electrical conductivity of the ceramic superconductive pattern.

[0022] The insulating material of the insulating layer of the multi-layer substrate of EP-A-0292 125 is preferably a glass/ceramic composite which can be sintered from a green sheet thereof at a temperature lower than a temperature used when annealing a superconductive ceramic material. If the temperature for sintering the insulating material from a green sheet thereof is too high, the superconductive property of the superconductive ceramic material is degraded during the firing of the green sheet of the insulating material. An example of such a glass/ceramic composite, i.e., a composite of a ceramic and glass, is a composition containing 20 to 50% by weight of alumina, 10 to 60% by weight of quartz glass, and 20 to 40% by weight of glass or crystallized glass, which can be sintered at a temperature lower than the temperature of firing the paste for the superconducting ceramic material, typically lower than 1000°C, preferably lower than 900 °C.

[0023] EP-A-0 292 125 discloses various processes for manufacturing a multi-layer substrate comprising the superconductive ceramic material.

[0024] Thus, one method comprises the steps of:

preparing green sheets of an insulating material, the green sheets having first and second main surfaces and holes extending through the green sheets;

forming a first pattern of a noble metal paste on the first main surface of each of the green sheets;

forming a second pattern of the noble metal paste on the second main surface of each of the green sheets in such a way that the second pattern corresponds to the first pattern onto which it is superposed in a later step of laminating the green sheets;

coating the inner surfaces of said holes of the green sheets with the noble metal paste;

forming an interconnection pattern of a superconductive ceramic material paste on the first pattern of the noble metal paste of each of the green sheets, the width of the interconnection pattern being narrower than that of the noble metal paste pattern;

filing the holes of the green sheets with the superconductive ceramic material paste;

laminating the green sheets, so that the interconnection patterns of the superconductive ceramic material paste are encapsulated with the noble metal paste and isolated from the green sheets by said metal paste; and

firing the laminate, whereby a multi-layer substrate comprising insulating layers and interconnection patterns of a superconductive ceramic material isolated from each other by the noble metal is obtained.

[0025] A second method for manufacturing a multi-layer substrate embodying this invention comprises the steps of:

preparing green sheets of an insulating material having two main surfaces and holes extending through the green sheets;

forming a first pattern of a noble metal paste on one of the main surfaces, of each of the green sheets;

coating the inner surfaces of the holes of the green sheets with the noble metal paste;

forming an interconnection pattern of a superconductive ceramic material paste on the first pattern of the noble metal paste of each of the green sheets, the width of the interconnection pattern being narrower than that of the noble metal paste pattern;

filling the holes of the green sheets with the superconductive ceramic material paste;

forming a second pattern of the noble metal paste over the interconnection pattern of each of the green sheets, the second pattern of the noble metal paste corresponding to the first pattern of the noble metal paste so that the interconnection pattern of the superconductive ceramic material is encapsulated with the noble metal paste;

laminating the green sheets having the interconnection pattern of the superconductive ceramic material encapsulated with the noble metal paste; and

firing the laminate, whereby a multi-layer substrate comprising insulating layers and interconnection patterns of a superconductive ceramic material isolated by the noble metal is obtained.

[0026] A third such method comprises the steps of:

preparing an insulating ceramic base;

forming a first pattern of a noble metal paste on the insulating ceramic base;

forming a first interconnection pattern of a superconductive ceramic material paste on the first pattern of the noble metal paste, the width of the interconnection pattern being narrower than that of the noble metal paste pattern;

forming a second pattern of the noble metal paste over the first interconnection pattern of the superconductive ceramic material paste so that the first interconnection pattern of the superconductive ceramic material paste is encapsulated with the noble metal paste;

forming a first layer of an insulating material paste over the second pattern of the noble metal paste and the insulating ceramic base, the first layer of an insulating material paste having holes extending through the first layer of the insulating material paste;

forming a third pattern of the noble metal paste on the first layer of the insulating material paste;

covering the inner surface of the penetrating holes of the first layer of the insulating material paste with the noble metal paste;

forming a second interconnection pattern of the superconductive ceramic material paste on the third pattern of the noble metal paste, the width of the interconnection pattern being narrower than that of the third noble metal paste pattern;

forming a fourth pattern of the noble metal paste over the second interconnection pattern of the superconductive ceramic material paste so that the second interconnection pattern of the superconductive ceramic material paste is encapsulated with the noble metal paste;

repeating the above steps from the formation of the first layer of the insulating material paste to the formation of the fourth pattern of the noble metal paste at least one more time; and

firing the laminate including the insulating ceramic base, whereby a multi-layer substrate comprising insulating layers and interconnection patterns of superconductive ceramic material isolated by the noble metal is obtained.

[0027] The following Examples illustrate the invention. In the Examples, reference will be made to the accompanying drawings, wherein:

Figure 1 is a flow chart of the manufacturing process of a multi-layer substrate of Example 1;

Figs. 2A and 2B are sectional view of multi-layer substrate of Example 1;

Fig. 3 shows the electrical resistivity of the superconductive ceramic material patterns of a multi-layer substrate of Example 1;

Fig. 4 is a flow chart of the manufacturing process of a multi-layer substrate of Example 2;

Fig. 5 shows sectional views of green sheets before lamination in Example 2;

Fig. 6 is a sectional view of a multi-layer substrate of Example 2;

Fig. 7 is a sectional view of another multi-layer substrate of Example 2;

Fig. 8 is a schematic view of a process of atomizing a superconductive ceramic material;

Fig. 9 illustrates the main steps of forming through-holes in a green sheet by causing balls to penetrate the green sheet of Example 4; and,

Fig. 10 is a perspective view of an electrical device in which chips are mounted onto a multi-layer substrate.

Example 1

[0028] A paste of a superconductive ceramic material was prepared by blending powders of BaC0₃ , Y₂0₃ , and CuO at a molar ratio of 0.6:0.2:1, and mixing the powders in a ball mill for 20 hours to homogenize the mixture. The mixture was pressed at 200 MPa to form pellets having a diameter of 30 mm. The formed pellets were then heat treated in air at 900 ° C for 6 hours and pulverized to form powders of Ba_o.₆Y_o.₄ CuO_4-δ. To these powders of Ba_o.₆Y_o.₄ CuO_4-δ were added a binder (e.g., ethylcellulose), terpineol and methylethylketone then kneaded in a paste mixer for about 10 minutes, and passed 30 times through a roll mill as a second kneading process. The resultant paste has a viscosity of about 2000 poise.

[0029] The superconductive ceramic material may be a Y-Ba-Cu-0 system, Sc-Ba-Cu-O system, or La-Sr(Ba)-Cu-O system compound, mentioned before. Preferably, the superconductive ceramic material is a superconductive ceramic material of the Y-Ba-Cu-0 system, for example, YBa₂ Cu₃O_7-δ, Y_o.₄Ba_o.₆ CU03 or (Y_0.6Ba_0.4)₂CuO_4-δ, which exhibits a superconductivity at a temperature of liquid nitrogen. The binder may be any organic binder, but ethylcellulose, polyvinylbutylalcohol, methylcellulose and acrylic acid-based polymer are preferred.

[0030] A green sheet of a glass-ceramic material was prepared as follows: Powders of alumina and glass having an average particle size of about 3 µm, a binder, and a solvent were blended and kneaded in a ball mill for 30 hours. The resultant slurry was tape-cast by the doctor blade method to form a green sheet having a dry thickness of about 300 µm. The thickness of a green sheet of a glass-ceramic material is preferably in a range of 100 to 500 µm, more preferably 200 to 300. The green sheet was cut into a square 150 mm x 150 mm, and the cut green sheet was perforated to form holes to be used as through holes

[0031] A composition for a green sheet or an insulating material may have the following compositions, as described in Japanese Unexamined Patent Publication (Kokai) No. 59-995, published on June 6, 1984:

[0032] From the slurry having the above compositions, a green sheet having a uniform thickness of, e.g., 0.3 mm, may be formed by the doctor blade method. Alternatively, the slurry may be used as a paste for printing.

[0033] Also, a glass ceramic composition of a mixture of 20 to 50% by weight of alumina, 10 to 60% by weight of quartz glass, and 20 to 40% by weight of glass or crystallized glass, which can be fired at a temperature lower than the temperature for firing the superconducting ceramic material, typically lower than 1000°C, may be used. (Japanese Unexamined Patent Publication (Kokai) No. 60-254697, published on December 16, 1985). An example of this composition has the following glass ceramic powder and slurry compositions:

[0034] This slurry can be cast to form a green sheet by the doctor blade method or used as an insulating paste for printing.

[0035] Onto the resultant green sheet, an interconnection pattern of the ceramic superconducting material paste was printed to a thickness of about 20 µm. The paste of a ceramic superconductive material may be replaced by a paste of powders of BaC0₃ , Y₂0₃ , and CuO, in a mole ratio of 0.6:0.2:1 with a binder, terpineol and methylethylketone The thickness of the interconnection pattern of the ceramic superconductive material is preferably more than 20 µm, more preferably 20 µm to 40 µm. The paste of the ceramic superconductive material also filled the holes of the green sheets completely through by the same printing step or by a separate step.

[0036] Thirty sheets of the dried printed green sheets were laminated and pressed at a pressure of 30 MPa. The laminate was then fired in air at 1000°C for 5 hours to form a multi-layer substrate composed of consolidated glass-ceramic insulating layers and ceramic superconductive layers. The firing atmosphere may be oxygen or an atmosphere having a lower partial pressure of oxygen than air. Then, the obtained multi-layer substrate is annealed in oxygen at 900 °C for 30 minutes.

[0037] Figure 2A illustrates the obtained multi-layer substrate 1 in which a plurality of glass-ceramic insulating layers 2 were laminated with, superconductive ceramic layers or patterns 3 inserted between the insulating layers 2 and through holes 4 formed to interconnect adjacent ceramic superconductive layers or patterns 3. As seen in Fig. 2A, an additional insulating layer 5 may be inserted between the insulating layers 2 where the superconductive ceramic patterns do not exist, by screen printing, if necessary.

[0038] The electrical resistivity of this multi-layer substrate was measured by the four probe method. The results are shown in Fig. 3. The electrical resistivity became zero at 77 K, the temperature of liquid nitrogen.

[0039] A multi-layer substrate similar to the above may be formed on a hard or sintered ceramic base such as an alumina plate. Figure 2B illustrates such a multi-layer substrate. Paste layers of a superconductive ceramic material 3 and an insulating material 2 were successively and repeatedly formed on an alumina plate 6 by a printing process such as screen printing, and then fired, and thus a multi-layer substrate having an alumina plate as a base was manufactured by a thick film forming method.

Example 2

[0040] The following description relates to Figures 4 to 6.

[0041] A paste of a superconductive ceramic material was prepared in accordance with the procedure of Example 1.

[0042] A gold paste was prepared by blending a gold powder having an average particle size of about 1 am, a binder, terpineol, and methylethylketone and kneading the blend in a paste mixer for about 10 hours, during which time the methyethylketone was completely evaporated. The kneaded blend was passed 30 times through a roll mill as a second kneading process, to obtain a gold paste having a viscosity of about 2000 poise. If the resultant gold paste has a viscosity higher than 2000 poise, a small amount of terpineol can be added to bring the viscosity to about 2000 poise.

[0043] The preparation of green sheets of a glass ceramic material was similar to that of Example 1. After square green sheets having a size of 150 mm x 150 mm and perforated with holes for through-holes were prepared, a first pattern of the gold paste 12 was printed on an upper surface of a first green sheet 11. The gold paste 12 preferably has a dry thickness in a range of less than about 10 µm, more preferably from 5 to 10 µm. The gold paste pattern had a first pattern 12 corresponding to a first interconnection pattern 13 of a superconductive ceramic material and having a pattern width larger than the width of the first interconnection pattern 13 of the superconductive ceramic material. For example, the first pattern 12 of the gold paste had a width of 100 to 200 am. In the printing of the gold paste, the gold paste was made to flow into the holes of the first green sheet 11 and cover the inner surfaces of the holes to form a gold layer 14. This flow of gold paste into the holes was effected by adjusting the viscosity of the gold paste. However, covering the inner surfaces of the holes with a gold paste may be effected by a process separate from the printing of the first pattern 12 of the gold paste onto the upper surface of the green sheet 11, if necessary. The first pattern 12 of the gold paste 12 and the gold layer 14 were then dried at 80 ° C for 10 minutes.

[0044] Subsequently, the first interconnection layer or pattern 13 of the paste of the superconductive ceramic material was formed on and within the first pattern 12 of the gold paste. This pattern 13 preferably has a thickness of at least 20 am, more preferably 20 to 40 am, and a width narrower than that of the first gold paste pattern 12, generally 50 to 150 µm, for example, 100 µm. The paste of the superconductive ceramic material also filled the holes of the green sheet 11 as shown at 15.

[0045] Preferably, a paste of an insulating material that is the same as the material of the green sheet 11 is printed on the first green sheet 11 where the first pattern 12 of the gold paste is not formed. The resulted insulating material paste pattern is denoted by reference numeral 16 in Fig. 5. Also preferably, a gold paste pattern is printed on the first gold pattern 12 along the peripheries thereof to isolate the superconductive ceramic material paste pattern 13 from the insulating material paste pattern 12. This additional gold paste pattern is denoted by reference numeral 17 in Fig. 5. These gold paste patterns 16 and 17 are preferably applied, in that they level off the top surface of the printed green sheet, but they are not essential.

[0046] Referring now to the second green sheet 11', second gold paste patterns 12' and 14', a second superconductive material interconnection pattern 13', through holes 15' of the superconductive ceramic material paste, a complemental insulating paste pattern 16', and an additional gold paste pattern 17' were formed on the upper surface and in the holes of the second green sheet 11', in accordance with the same procedures as for the first green sheet 11. However, a pattern 18 of gold paste was printed on the lower surface of the second green sheet 11' before the superconductive ceramic material paste filled the holes. The pattern of the gold paste pattern 18 corresponded to the first gold paste pattern 12 so that, when the second green sheet 11' was superposed on the first green sheet 11, the gold paste patterns 12 and 18, and if included, the gold paste pattern 17, encapsulated the first interconnection pattern 13 of the superconductive ceramic material paste. A complemental pattern 19 of the insulating material paste can be preferably printed on the lower surface of the second green sheet 11', to level off the surface thereof.

[0047] In accordance with the same procedures as for the second green sheet 11', a total of 30 green sheets including the first green sheet 11 were provided with necessary patterns of gold paste, the superconductive ceramic material paste, and the insulating material paste.

[0048] These 30 green sheets were laminated and pressed at a pressure of 30 MPa. Figure 6 is a sectional view of the resulting laminate, in which reference numeral 21 denotes the insulating green sheets, 22 denotes the superconductive ceramic material patterns or layers, 23 denotes the gold paste patterns or layers, and 24 denotes the insulating material patterns or layers. In Fig. 6, patterns 25 and 26 of the superconductive ceramic material paste are separated by a pattern 28 of the insulating material paste, while the patterns 26 and 27 of the superconductive ceramic material paste are connected through the pattern 29 of the gold paste, and are not connected by the superconductive ceramic material paste itself. The latter structure may be changed if desired.

[0049] The laminate of Fig. 6 was then fired in air at 1000°C for 5 hours to form a multi-layer substrate in which the insulating material sheets, gold layers, and superconductive ceramic material layers are made into a monolith. The laminate is then annealed in oxygen at 900 ° C for 30 minutes. The firing conditions should be such that the insulating material (a glass/ceramic material in this example) is sintered and the superconductive ceramic material pattern becomes superconductive for current passage. Generally, the superconducting ceramic material of the X_aY₁-_aZO_b or (X_1-aY_a)₂ZO_b requires firing in an atmosphere containing oxygen such as in air or in oxygen at about 850 to 1000°C, typically 950 ° C for 1 to 30 hours, typically 5 to 12 hours. After firing, it is preferable to effect annealing in oxygen at 800 to 900 ° C for 10 to 120 minutes, typically 30 minutes. The superconducting ceramic material of the bismuth-system such as BiSrCaCu₂0_x requires firing in an atmosphere containing oxygen, usually in air at 850 to 900 C, typically 870 to 880 ° C for 1 to 30 hours, typically 12 hours. If the temperature of the firing and annealing of the superconductive ceramic material is too high, the superconductivity of the material is degraded or even lost. Therefore, the insulating material of the green sheet must be able to be fired at a temperature lower than the firing or annealing temperature of the superconductive ceramic material; i.e., a glass ceramic material which can be fired or sintered at a relatively low temperature is preferable. The atmosphere in which the laminate is fired may be air, an atmosphere containing oxygen in an amount of at least 5%, oxygen.

[0050] The electrical resistivity of the obtained multi-layer substrate was measured by the four probe method, and showed results similar to Example 1 and Fig. 1, including a zero resistivity at the temperature of liquid nitrogen (77 K).

[0051] The gold may be replaced by other noble metal such as silver, platinum or an alloy thereof, since these metals are inactive in an oxygen-containing atmosphere at a temperature of about 1000°C and act as a barrier layer for preventing a diffusion of ingredients of the superconductive ceramic material into the insulating material or the glass ceramic material. Copper, for example, is oxidized in an oxygen-containing atmosphere at about 1000°C, to form a copper oxide which does not prevent a diffusion of the ingredients of the superconductive ceramic material therethrough. Superconductive ceramic materials are sensitive to glass ceramic materials. If the ingredients of the superconductive ceramic material diffuse into a glass ceramic material, the superconductive ceramic material is degraded and the superconductivity thereof may be lost. Therefore, it is preferable to encapsulate the patterns of the superconductive ceramic material with a noble metal such as gold, silver, platinum or an alloy thereof, in a multi-layer substrate.

[0052] The metal encapsulating the superconductive ceramic pattern may act as a supplemental or complemental conducting pattern, because metals such as gold, etc., have a high electrical conductivity. When a supplemental or complemental conducting pattern only is desired, and encapsulation of the superconductive ceramic material is not necessary, the metal layer or pattern may be formed so that it is in contact with only one side of the superconductive ceramic material layer or pattern; i.e., it is not necessary to encapsulate the superconductive ceramic material pattern.

[0053] Returning to Fig. 5, the gold paste pattern 18 may not be formed on the second surface of the green sheet 11' or 11" and may be formed over the first interconnection pattern 13 of the superconductive ceramic material of the first green sheet 11. In this case, the insulating material pattern 19 can be a part of the insulating material pattern 16, and need not be printed separately therefrom. Accordingly, all of the printed layers may be formed on one side of a green sheet, and lamination of the green sheets and firing of the laminate may be carried out as described before.

[0054] The multi-layer substrate as described above may be formed on a hard or sintered ceramic base such as an alumina plate 30, as seen in Fig. 7. In this case, not only the patterns of the gold 23, the superconductive ceramic material 22, and the insulating material 24, but also the insulating layers 21' instead of the green sheets 21, are formed by a thick film forming method such as screen printing.The thickness of the insulating material layers 21' is generally more than 80 µm, preferably from 100 to 150 µm, for example, 120 I.Lm.

Example 3

[0055] Example 2 was repeated except that, in the preparation of the superconductive ceramic material paste, the powder of CuO was replaced with a metal powder of copper and the superconductive ceramic material was formed at the stage of firing the laminate of the green sheets.

[0056] Powders of BaC0₃ , Y₂0₃ , and Cu at a molar ratio of 0.6:0.2:1 were mixed with a binder, terpineol, and methylethylketone and kneaded in a paste mixer for 10 hours, followed by passing the mass 30 times through a roll mill. Thus, a paste having a viscosity of about 2000 poise was obtained.

[0057] This paste was used to form patterns of a superconductive ceramic material. The procedures for forming a laminate of the green sheets were the same as in Example 2.

[0058] The firing of the laminate was conducted in air, oxygen, or a mixture of oxygen and an inactive gas, at about 950 C for 12 hours. The annealing of the laminate was conducted in oxygen at 850 C for 60 minutes. After firing, a multi-layer substrate is obtained and the electrical resistivity of the superconducting ceramic material patterns of the multi-layer substrate was measured by the four probe method, which showed a result similar to Fig. 1, including a zero resistivity at 77 K, i.e. liquid nitrogen temperature.

Example 4

[0059] Example 1 was repeated, except that the through holes were formed by causing balls of a superconductive ceramic material to penetrate the green sheets.

[0060] The balls of a superconductive ceramic material were prepared as shown below. For example, referring to Fig. 8, BaC0₃ , Y₂0₃ , and CuO were mixed at a molar ratio of 0.6:0.2:1 and fused in a crucible 31 heated by a heater 32. The fused ceramic material 33 was dropped through a nozzle 34. A high pressure gas 36 was applied onto the dropping stream of the fused ceramic material 35 through a spraying nozzle 37, and thus the fused ceramic material was atomized and cooled to form powders of the ceramic material 38. The carrier gas for the high pressure gas 36 was, for example, N₂ , Ar or air, and the pressure of the gas was about 3 to 5 kg/cm². Accordingly, spherical balls of superconductive ceramic material having a particle size of 20 to 500 µm were obtained by this atomizing process.

[0061] The penetration of the balls of the superconductive ceramic material was conducted as shown below. A green sheet 41 of a ceramic material was placed on a plate 42 having recesses 43 where the through holes were to be formed. Another plate 44 having tapered holes 45 where through holes were to be formed was placed on the green sheet 41, the tapered holes 44 being located just above the recesses 43 of the underlying plate 42. Balls 46 of a superconductive ceramic material were inserted in the tapered holes 45. A punching board 47 having protrusions 48 was set above another plate 44 having the holes, in such a manner that the protrusions 48 corresponded to the holes 45. The assembly of the underlying plate 42, the green sheet 41, the other plate 44 and the punching board 47 was then heat pressed and thus the balls 46 were made to penetrate the green sheet 41, whereby a green sheet 41 in which the through holes were formed by the balls 46 of the superconductive ceramic material was obtained. This process is described in Japanese Examined Patent Publication (Kokoku) No. 58-32797.

[0062] By this ball penetration method, a precise perforation and filling of the holes can carried out in one step.

[0063] The balls of superconductive ceramic material may be balls of an unfired superconductive ceramic material. For example, a paste of a superconductive ceramic material as described in Example 1 can be formed by a mold into spherical balls, in which a heat curing resin was used as a binder for the paste.

Example 5

[0064] In the procedure of Example 2, a coupling agent such as titanium or silicon coupling agent was included in the metal encapsulating the superconducting ceramic material pattern, to improve the adhesion of the metal with the green sheet, particularly the green sheet of a glass ceramic material, as well as with the superconducting ceramic material pattern.

Example 6

[0065] A multi-layer substrate as in the above Examples was manufactured, and IC's, LSI's, VLSI's, Josephson elements, etc., were mounted on the multi-layer substrate, as shown in Fig. 10, wherein reference numeral 51 denotes the multi-layer substrate, 52 denotes superconductive ceramic material patterns, 53 insulating layers, and 54 chips or elements such as IC's, etc.

Claims

1. A paste for forming a superconducting ceramic film, comprising a powder of a superconducting ceramic material or powders of ingredients which form a superconductive ceramic material by firing, an organic binder and a solvent, characterized in that the superconductive ceramic material is an oxide ceramic material represented by one of the general formulae X_aY_1-aZO_b and (X_1-aY_a)₂ZO_b, where 0 < a < 1; 3 b b < 4; X is at least one element selected from the group of Ba, Ca, Sr, and Mg; Y is at least one element selected from the group of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; and Z is at least one element selected from the group of Cu, Ag, and Au.

2. A paste for forming a superconducting ceramic film, comprising a powder of a superconducting ceramic material or powders of ingredients which form a superconductive ceramic material by firing, an organic binder and a solvent, characterized in that the superconductive ceramic material is an oxide ceramic material represented by the formula {M^IIO)_x(M₂^IIIO₃)_1-x}_y(CuO)_z(O)_δ, wherein M" is at lest one of Ba, Sr, Ca, and Mg; M^III is at least one of Y, Sc, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, and Lu; 0.5 < x 0.9; 1 ≦ y 2; 1 ≦ z 2; and 6 is a deviation of the amount of oxygen from the stoichiometry; the oxide ceramic material exhibiting superconductivity above the boiling point of nitrogen.

3. A paste according to claim 1 wherein the superconductive ceramic material is YBa₂Cu₃O_7-δ.

4. A paste for forming a superconducting ceramic film, comprising a powder of a superconducting ceramic material or powders of ingredients which form a superconductive ceramic material by firing an organic binder and a solvent, characterized in that the superconductive ceramic material is one of superconducting oxides of Bi-Sr-Ca-Cu-O and TI-Bi-Sr-Ca-Cu-O systems.

5. A paste according to claims 1 to 4 wherein the organic paste is selected from the group consisting of ethylcellulose, polyvinylbutylalcohol, methylcellulose, and acrylic acid-based polymer.

6. A paste according to any preceding claim, which comprises, for each 100 parts by weight of said powder, 0.5 to 10 parts by weight of the organic binder and 5 to 10 parts by weight of the solvent.

Drawing